This invention relates to time division switching systems and in particular to circuits for providing conference call service in time division switching systems.
Many electronic switching systems utilize a switching technique which is known as time division switching. In this technique, communicating stations are connected to a common bus for short time intervals called time slots in order that the stations may exchange information. Typically, such systems have an incoming time division bus which receives output signals from the connected stations and an outgoing time division bus from which connected stations receive input signals. The two time division buses are connected by a summing amplifier circuit so that signals placed on the incoming bus are summed together and appear on the outgoing bus. Each station which is connected to the bus places its outgoing signal on the incoming bus, which signal is added to that of the other connected stations. Each station then receives the sum of its own and the other station signals from the outgoing bus. The stations contain a subtraction circuit which removes the signal contribution due to their own signals from the incoming sum signal. Therefore the station receives only the signals generated by the other connected station. At the end of a time slot, the stations are disconnected and another group of stations is connected and the process is repeated. In this way, many stations may be sequentially connected together to perform a communications switching function. The time slots occur in repetitive cycles so that at the end of a system cycle, the connections are repeated in sequential fashion.
Many of the switching systems available today contain a conferencing call feature. This feature allows three or more stations to be simultaneously connected together so that each may converse with the others. Many prior art circuits which perform the conferencing function simply connect each station in the conference call to the time division buses in a single time slot which is dedicated to the conference call. Each station then receives from the outgoing bus a composite signal which is the sum of all signals generated by all of the stations in the conference call. The station subtracts its own signal from the composite sum and receives the signals generated by the other stations. It has been found, however, that such conferencing circuits generate excessive transmission imbalance. The imbalance results when many stations are connected to the time division buses simultaneously and the summing bus amplifier becomes overloaded. The overloaded amplifier produces output signals on the outgoing bus which are of reduced magnitude; thus when each station subtracts its own signal from the reduced amplifier output, its own signal contribution is undesirably overcompensated.
Other circuits have been devised which utilize a transfer arrangement implemented by constant current sources which are connected to a storing circuit for a time proportional to the magnitude of the signal generated by the connecting stations. The composite sum which is stored is then sent to each station. Such circuits typically involve extensive and complicated circuitry to provide the conferencing function.
It is apparent therefore that there is a need for a conferencing circuit which minimizes transmission imbalance without requiring extensive and complex circuitry.
Accordingly, it is an object of the present invention to provide a conferencing circuit which reduces problems caused by overload of a summing bus amplifier.
It is a further object of the present invention to simplify the circuitry necessary for conferencing circuits.